1. Field of the Invention
This invention relates generally to a contact-type panel input device. More particularly, the present invention relates to touch panel input devices for which materials high in vibration-absorption characteristics, such as, for example fingers and felt-tipped pens, are utilized for input.
2. Description of the Related Art
Touch panels utilized as input devices in conjunction with materials high in vibration-absorption for input are known. Such high vibration absorption materials include fingers, felt-tip pens and the like.
An example of the touch panel device is an ultrasonic wave touch panel, such as that described in the publication of unexamined Japanese patent application No. 61-239322, a description of which is given below. Reference is now made to FIG. 14. FIG. 14 illustrates a block diagram of a of touch panel device 100. In this configuration, processor or CPU 7 controls transmission circuit 2 for electrically generating oscillation signals. The electrical oscillation signals are then converted into mechanical vibrations in the X-direction by X-direction emission transducer 22 which is located on panel 1. Similarly Y-direction transducer 20 generates vibration in the Y-direction. This results in surface acoustic waves being sent in panel 1 and reflected by reflective arrays 15, 16, 17, and 18, provided along the vertical and horizontal axes in the circumferential portions thereof. The transmitted surface acoustic waves possess rectilinear propagation characteristics and thus advance in a roughly linear manner. Some of the surface acoustic waves propagating rectilinearly are reflected 90.degree. by being reflected by reflective element 1 of X-direction reflective element array 17 of the panel, while the remainder pass through the element. The surface acoustic waves pass through one element of array 17 receive similar treatment by the next element of X-direction reflective element array 17. In this way, the surface acoustic waves that have been reflected proceed perpendicularly to X-direction reflective element array 17 of the panel, with time differences proportional to their route differences. Surface acoustic waves possessing time differences proportional to their route differences are again reflected by X-direction reflective element array 18 and are reflected 90.degree.. The multiple surface acoustic waves that have been reflected twice go through the same route and are received by X-direction reception transducer 23. The mechanical vibrations received by X-direction reception transducer 23 in this way are converted to electrical signals and are then sent to receiving circuit 3. The received signals are amplified by amplifier 4 and are detected by wave detector 5. In this way, the electrical signals output by X-direction reception transducer 23 are converted to a waveform shown in a solid line in FIG. 13. This waveform is quantized by analog-to-digital (A/D) converter 6 and is stored in RAM 9.
FIG. 4 shows an example of a waveform received when no touch input is being received. In the figure, the horizontal axis shows time, and the output of A/D converter 6 has the distribution as shown. As shown in FIG. 13, because the time taken by individual signals differs depending on the length of the route a signal takes after being output by the emission section and before being received by the reception transducer. FIG. 5 shows a received waveform received when a touch input is being received. Because the surface acoustic waves passing through the spot of the panel touched are attenuated due to absorption by the object that touched the panel, the level of the receive signal that corresponds to the route passing through the spot at that time will be smaller. It is possible to determine the position at which the panel was touched, by storing the signal received when no touch input is being received in RAM 9, and comparing it to the signal received when a touch input is being received. The position at which the panel was touched is the point at which the magnitude of attenuation becomes the largest, i.e. using a peak detection method. Therefore, to accurately determine this position, either the sampling interval can be shortened, or zero-cross can be detected by differentiating the waveform received. The above explanation concerns positional detection in the X direction. By performing similar detection in the Y direction, it is possible to determine the X and Y coordinates of the position at which the panel was touched. Here, the basic resolution of a touch panel greatly depends on the sampling interval, and can be expressed using the equation shown below, where t is the sampling interval and V is the propagation velocity. EQU Resolution=t.multidot.V/2 Equation 1
Furthermore, taking into account sampling errors, actual resolution can be expressed using the equation shown below. EQU Resolution=t.multidot.V Equation 2
Moreover, in some methods, surface acoustic wave attenuation corresponding to the positions detected in the X and Y directions is treated as the detection value in the Z direction at the touched position, and touching pressure is determined based on this detection value.
The stronger the touch pressure is the wider becomes the area of a touched spot and the more likely it becomes that the received signal does nor exhibit a clear maximum attenuation value. Instead, the attenuation gets saturated and a flattened attenuation peak results. Under such conditions it is not possible to accurately determine the touch position by means of the peak detection method. Furthermore, any noise peak superimposed to the flattened attenuation peak tends to be misinterpreted as the attenuation peak leading to incorrect detection results. For the same reason, it was difficult to use a pen having a felt tip or the like as a contact object, since, due to the flat contact surface of the tip of such pen, a peak attenuation value is hardly obtained.
In a conventional position detection device there are other known problems. For example. the resolution of the touch panel is greatly affected by the sampling interval (t) used for received signals and overlapping noise. FIG. 13 shows the relationship between the variation in the detected position and the attenuation waveform significantly affect such detection. This figure shows the difference between the waveform when the panel is not being touched and that when the panel is being touched, for a case in which there is a sufficient quantity of noise. Curves a, b, and c indicate attenuation waveforms when the width of the area touched is varied. In FIG. 13, the broken lines indicate the variation in the attenuation waveform caused by noise, and the bold lines indicate the width of the variation in the detected positions when the peak detection method is used to detect the touched positions. If resolution is defined as the variation in the detected positions, the length of the bold lines indicate resolution. As can be seen in FIG. 13, when the peak detection method is used for position detection, the peak value becomes saturated when the width of the area touched is wide, as indicated by a, resulting in inferior resolution. Furthermore, a sharp attenuation waveform cannot be obtained even when the width of the area touched is narrow, as indicated by b, again resulting in inferior resolution. As can be seen, a problem exists in methods that regard the peak position as the touched position, i.e., the touched position cannot be accurately determined. Furthermore, if noise elements exist in the received signal waveform, resolution cannot be improved even if sampling interval t is shortened. The document U.S. Pat. No. 4,700,176 mentioned above discusses an alternative way of detecting the touch position from the signal received from the output transducer. In this case the detected (rectified) signal from wave detector is differentiated and the zero-crossing of the differentiated signal is detected as corresponding to the touched position. As mentioned in the document itself, this detection method is vulnerable to interpreting noise components in the signal as a "touch". Thus, this detection method is no more reliable than the peak detection method.
The conventional touch panel input devices are intended to be used as "pointing devices", i.e., it is detected, for instance, whether, and if so which, one of plural points offered for selection by a computer display screen disposed behind the panel has been touched, for instance by a finger. While for this purpose ("finger input" or "pointing input") the accuracy in position detection achieved by the methods explained above may be sufficient, it is not sufficient where the touch panel input device is to be used for "pen input" ("line input"), i.e. for drawing characters, symbols or graphic figures. In this case lines of contiguous points on the panel are touched by moving a contacting object on the panel surface. In this case the touch pressure will hardly be constant and the width of the touched positions will fluctuate correspondingly. As will be understood from the foregoing, this fluctuating touch width affects the accuracy of position detection. On the other hand, where for instance a character is drawn on the panel the shape and position of lines making up the character must be exactly detected to allow data to be derived from the panel that are suitable for being processed by an information processing device for display of the character on a screen or printing it by means of a printer. In other words, "pen input", as defined herein, to a touch panel input device requires a much higher accuracy of position detection than just "finger input".